Uv induced afterglow and thermoluminescence in eu-doped alkali halides at low temperature

Abstract

This work is focused on the study of the mechanisms involved in the afterglow (AG) and the thermoluminescence (TL) phenomena in Eu-doped alkali halides exposed to low­ energy UV radiation at low temperature. Our study of the radiation induced defect formation and defect recombination is based on the excitonic theory. In pure alkali halides, this theory establishes that the primary products of irradiation are F centers and H centers. The process of formation of these defects from free electrons and holes implies the formation of self-trapped excitons (STEs) which relax to an off center configuration similar to nearest neighbor F-H defect pairs. The formation of a stable defect pair would be due by the migration of the H center far enough from the F center position. In Eu-doped alkali halides, the formation of stable defects responsible for AG and TL phenomena could be observed after irradiation with Ulight of energy close to the high­ energy absorption band of the Eu-impurity. It was considered that UV irradiation at low temperature induces the formation of a STE in the vicinity of the impurity which results in the formation of an F-H pair. We propose that the migration of the H center occurs along dislocation lines. This model predicts that the AG decay follows a r3 2 law after pulsed irradiation. Our experimental work is developed in a research facility designed to perform systematic studies of PL, AG and TL after irradiation with UV-visible light at temperatures from 20 K to 300 K. The Eu-doped alkali halide samples were submitted to different thermal treatments which are known to be efficient for the precipitation of the impurity in different phases. The participation of the dislocation lines in the AG and TL phenomena was analyzed by exposing the samples to deformation treatments which results in the movement and the formation of dislocations. The experiments show a higher participation of Eu-aggregates in the AG and TL process than Eu-vc dipoles. Furthermore, it is shown that the participation of dipoles is much higher in deformed crystals. This results allow us to propose that: (1) radiation induced defect formation at low temperature occurs mainly around dislocations because they may serve as guiding lines for the H centers to separate from the F center to form stable F-H center pairs. Here it is assumed that the migration of H centers in dislocation lines is fast compared to the migration in the bulk at temperatures too low to allow the H-center migration through the bulk; (2) aggregation of impurity ions occurs mainly near dislocations because, firstly, impurity ions migrate faster along dislocation lines causing the aggregation rate to increase and secondly the impurity ions are trapped into dislocations where they remain mobile along the dislocation.